feat: createIfMissing option, named exports, README overhaul, remove internal docs

[Copilot]

Several code quality issues: incorrect default documented for saveDelay, raw ENOENT on missing files, no named exports for ESM/TS consumers, and internal working-note .md files cluttering the repo root.

Changes

app.js

• createIfMissing config option (default: false) — when true, load() initializes contents to {} and writes the file instead of propagating ENOENT
• Named exports — adds module.exports.Filejson and module.exports.default alongside the default export

// Now works in CJS and ESM
const { Filejson } = require('filejson');
import { Filejson } from 'filejson';

// Auto-create missing config files
const file = new Filejson({ createIfMissing: true });
const instance = await file.load('settings.json'); // creates {} if absent

index.d.ts

• Switched from export = Filejson to export { Filejson }; export default Filejson; for true ESM named-import support
• Added createIfMissing?: boolean to Config

README.md

• Removed outdated Requirements section (Node.js version table, legacy Node 5 section)
• Async/Await example now leads; callback and promise examples follow
• Fixed Recommended Delays table — 0ms was incorrectly marked as the default; 100ms is the default

Deleted internal docs

Removed 6 *-IMPLEMENTATION-SUMMARY.md files (ATOMIC-WRITES, DEBOUNCE, ESLINT, IMPLEMENTATION, NPM-DISCOVERABILITY, TEST) that were internal working notes with no value to consumers.